Doctor process using air and sulfur



March 1955 M. L. KALINOWSKI ET AL 2,739,925

DOCTOR PROCESS USING AIR AND SULFUR Filed Sept. 22, 1953 SEPARA 70/? T I I g -1. & n:

E e 7 Q E 9') INVENTORS= Mai/raw L. Iua/inowslri BY Russell Bray/r AT OHW DOCTOR PROCESS USING AlR AND SULFUR Mathew L. Kalinowski, Chicago, 111., and Russell H.

Brown, Hammond, Ind., assignors to Standard Oil Company, Chicago, 111., acorporation of Indiana Application September 22, 1953, Serial No. 381,542 11 Claims. (Cl. 196-33) This invention relates to the sweetening of sour hydrocarbon oils. More particularly the invention relates to an improvement in the sweetening of sour petroleum distillates by the doctor process.

Owing to the malodorous nature of the mercaptans' present in most petroleum distillates, it is necessary to remove these compounds in order to obtain a salable material. Many methods are known for removing the mercaptan odor from mercaptan-containing distillates. These mercaptan-containing distillates are commonly known as sour distillates and the essentially mercaptan-free distillate s are commonly known as sweet 'distillates. The more common processes in use-in the petroleum industry convert the malodorous mercaptans to disulfides which are relatively innocuous with respect to odor.

The oldest and probably most common sweetening process now in use is the so-called doctor process wherein an aqueous caustic-sodium plumbite solution and free-sulfur are used to convert the mercaptans to disulfides. The doctor process has many disadvantages which arise primarily from the formation of insoluble lead sulfide. Frequently extremely prolonged settling times are required to remove the PbS from the hydrocarbon oil. Normally not only the doctor solution but also a black-strap layer must be separated from the oil in order to get a PbS- free product oil. The black-strap layer which is a fairly stable emulsion of doctor solution, PbS and oil can be separated only by long settling times or by filtration. Some of the separation difiiculties in the doctor process can be overcome by the use of free-sulfur in marked excess of the theoretical amount of 0.5 mols per mol of mercaptan sulfur present in the oil. However, too large a sulfur addition results in a corrosive sweet oil.

An additional problem in the use of the doctor process lies in the regeneration of the spent doctor solution for reuse in the process. Th'e'doctor solution must be regenerated in order to provide a commercially economic operation. The spent doctor solution is regenerated by contact with free-oxygen at elevated temperatures, e. g., 250 to 350 F.

Despite these disabilities the doctor .process is used because for distillates boiling in the heavier-than-gasoline range it is often the only practical way to sweeten particular sour oils.

An object of the invention is to sweeten sour hydrocarbon oils by means of the doctor process. Another object is to sweeten sour petroleum distillates which boil in the heavier-than-gasoline range, i. e., between about 325 and 650 F. A particular object is the sweetening of a sour oil by adoctor process wherein the conventional separate spentdoctor regeneration procedure is eliminated. Other objects will become apparent in the course of the detailed description. g Y

It has been found that it is possible toeliminate the separate regeneration procedure of the conventional doctor process by contacting the free-suIfur-sour' oil doc not only to sweeten the sour oil but also to convert the PbS formed to water-soluble plumbite; at the end of this time the aqueous phase is separated from the sweet oil and is recycled to the sweetening zone.

The hydrocarbon oil feed to the process of this invention may be any liquid hydrocarbon oil containing detectable amounts of mercaptans, i. e., the oil should be sour to the conventional doctor test or should have a mercaptan number or copper number above about 1. The process is particularly suitable for the treatment of sour petroleum distillates boiling below about 750 F. Examples of these distillates are naphtha, kerosene, diesel oil, heater oil and gas oil. The process may be used on distillates obtained by the fractional distillation of crude petroleum or on distillates obtained from various conversion processes such as thermal cracking, catalytic cracking, reforming in the presence of hydrogen, etc. Sour petroleum distilla'tes boiling in the heavier-than-gasoline range between about 325 and 650 F., e. g., a heater" oil, are a preferred feed.

The sweetening agent of this invention consists of a' conventional doctor solution. The doctor solution is made up of an aqueous solution of an alkali metal hydroxide and the reaction product of litharge and the alkali metal hydroxide. The amount of free-alkali metal hyroxide present in the doctor solution may be between about 5 and 30 weight percent, usually between about 10 and 15%. The plumbite content is commonly given in terms of the percent of PbO theoretically present. This content for a fresh doctor solution is usually between about 1.5 and 2.5% based on aqueous caustic solution. However, it is to be understood that the process is operative with any range of doctor solution compositions that are operable in a conventional doctor process.

In order to obtain a sweet product it is necessary to have present in the agent-sour oil contacting zone free- "sulfur in an amount in excess of the theoretical.

The theoretical quantity of free-sulfur needed for the sweetening reaction is 0.5 mol per each mol of mercaptan sulfur present in the sour oil. It is generally known that the use of the theoretical quantity of free-sulfur does not produce an oil that is sweet to the doctor test. The amount of excess free-sulfur necessary varies with the sour oil charged and the operating conditions. In general the higher boiling the sour oil, the more excess sulfur that is needed to produce a sweet product. However,

' the amount of sulfur present should be controlled to a level tor solution mixture with free ox'yg'enata temperature 1 below that at which corrosive sulfur appears in the product oil, i. e., corrosive sulfur as determined by the copper strip test. In general the amount of sulfur added in excess of the theoretical amount (hereafter referred to as apercentage of the theoretical quantity, e. g., 10% excess sulfur, is equal to of the theoretical) may vary from 5% or less to as much as 100% or more.

In order to obtain the substantially simultaneous regeneration of the doctor solution and the sweetening of the. sour oil, it is necessary that free-oxygen be present in the contacting zone. The free-oxygen may be introduced in the form of a readily reducible compound such as permanganate, hydrogen peroxide, free-oxygen itself,'or in the form of atmospheric air. The amount of free-oxygen present in the contacting zone must be at least enough to oxidize the PbS formed. More than this amount is desirable. It is preferred to use between about and 250% of the theoretical requirement.

The temperature at which the mixture of sour oil,

doctor solution, oxygen and free-sulfur in the contactature between about 150 and F.

V The contact time needed in the contacting zone is" determined by the rate at which the PbS conversion" takes place. The higher the temperature the shorter the time needed. At temperatures approaching 200 F. the rates of sweetening and PbS conversion are substantially identical. 140 R, the total contacting time is about 50 minutes; at about 170 F, the time is about 15 minutes. The contacting is influenced by the amount and type of agitation in the contacting. In general when operating with temperatures between about 150 and 175 F., the contacting time will be between about 25 minutes and 10 minutes, wherein the longer times correspond to the lower temperatures, to obtain essentially complete conversion of PbS to the soluble plumbite form.

The annexed drawing which forms a part of this specification shows an illustrative embodiment of one particular method of utilizing the process of this invention. It..is to be understood that the drawing is schematic. in nature and many items of process equipment have been omitted since these may be readily added thereto by those skilled in this art.

In the drawing, sour oil from source 11 is passed by way of line 12 into heat exchanger 13. Herein the sour oil is a heater oil boiling between 330 and 570 F. with a rnercaptan number or" 65. This sour oil is derived by distillation from a high sulfur Texas crude. Herein the sour oil is substantially HzS-free and no prewash with aqueous caustic solution is carried out. When the sour oil contains appreciable amounts of H28 it is desirable toprewash the oil with aqueous caustic solution.

In heat exchanger 13 the sour oil is raised to a temperature high enough to provide a contacting zone temperature of about l7ll P. The hot sour oil is passed from heat exchanger 13 into line 14. A small amount of the hot sour oil is withdrawn from line 14 and is passed by way of valved line 16 into sulfur drum 17.

Sulfur drum 17 is a vessel filled with free sulfur, such as roll sulfur. The hot sour oil dissolves some of this sulfur and the free-sulfur-containing oil is passed from sulfur drum 17 by way of valved line back into line 14 where it. meets the remainder of the hot sour oil. In this illustrationsufiicient free-sulfur is dissolved to have present in the sweetening zone about 50% more free-sulfur than the theoretical amount, i. e., 50% excess sulfur (a sulfur usage of 150%).

The doctor solution from source 21 is passed by way of valved line 22 into line 23. At the start of a run only fresh doctor solution will be present in line 23. As the run proceeds the addition of doctor solution will be limited to makeup quantities. The amount of doctor agent is at least enough to form a separate aqueous phase. In general between about 1 and 20 volume percent based on v.sour oil is used. In this illustration volume percent ofdoctor solution is used. The fresh doctor solution is composed of about 12% of free sodium hydroxide, about 2% oflbO and the remainder water.

Air from source 26 is passed by way of line 27 into line 23. The amount of air. present must be at least enough to oxidize the PbS formed in the reaction, i. e., 2 mols of free-oxygen per mol of PbS formed. Herein about 200% of free-oxygen, based on the theoretical requirement, is added.

The doctor solution and the air in line 23 are introduced into line 14 at a point beyond the entry of the free-sulfur-containing oil. The doctor solution, air, sour oil, and free-sulfur are introduced into reactor 2? Reactor 28 is a vertical cylindrical vessel provided with l The stirrer in Although a .rncchanically agitated reactor is shown hereimit. is to be understood that other methods of 0btaining intimate contacting may be used, e. g., the reactormay be agitated with Also. an orifice mixer which provides a sufiicient contacting time may be used.

When operating at a temperature of about h contents .Q. 9, .21 a e maintained at a perature of about 170 F for 15 minutes in order to insure sweetening and essentially complete conversion of the PbS. Sweet oil and aqueous phase are withdrawn at a trap-out point near the top of reactor 28 and are passed by way of line 34.into separator 36. Separator 36 is a substantially horizontal cylindrical vessel providing sulficient settling'time for the formation of a separate aqueous phase and an oil phase. Separator 36 is provided withia vent 37 for the withdrawal of gases.

The loweraqueous phase is withdrawn from separator 36 by way of 'line'39. Normally the aqueous phase in line 39 is recycled to reactor 28 by way of lines 23 and 14. The conversion reaction results in the formation of water and sodium sulfate which dilute the doctor solution. Periodically this doctor solution is withdrawn from the system by way of line 39 and valved line 41.

The-oil phase withdrawn :from' separator 36 :by way of. line :44. .Thisi-oilaphase-contains a very slight :amount of occluded-aqueousiphase. =Thexoccluded solution may be removed from the sweet oil .by either Water washing orhy ;coalescing.' Hereimtheroil phase is passed from line rid-into ,coalescer g46.

Coalescer 46is ta vertical cylindrical vessel provided with glass wool. Instead of glass wool, coalescer may be filledwith :crushed rock as sand. Sweet haze-free product oil is withdrawn .from the top of coalescer 46 and is passed to storage, not shown by way of line 48.

The solution separated incoalescer 46 is withdrawn from-the bottomgthereofby'way ofline 51. This solution may .he recycledrto reactor 28 by way of valved line 52, line 39, .etc.;,-or it-may be withdrawn from the system -by.:way.of valved line-54.

l't'istobe understood that. the embodiment described aboveis -merely one illustr ation of the use of the process ofzthisinvention'inthesweetening of a sour hydrocarbon oil Many variations of the above will be immediately apparent to those skilled in the doctor sweetening art.

The results obtainable withthe process of this invention are-illustrated .byeigperimental data set out below.

All;theyexperirnents described hereinafter were carried out as follows: The contacting zone consistef of a 3- neclced glass flask with a capacity of 2 liters. The flask was provided with amotor-drivenstirrer, a thermometer andan electrically heated jacket. The sides of the flask were creased to improve agitation. One-half liter of Hzsefree souroil and fifty ml. of doctor solution were usedin eachrun. The doctor solution contained 11% NaOHand 1.8% RbO. ,The experiments were carried out byadding the .souroil and thedoctor solution to the flask in that order. Stirring was begun immediately after adding thesour oil; After the initial. contact of sour oil and doctor solution, 50%. excess free-sulfur was added to the flask-in .the form of a 1% solution of xylene. Air was bubbled-throughthe flask from the time of doctor solution-addition. .Tlj1.c .stirr-ingwas continueduntil the H38 conversion reaction was completed. The stirrer was then stopped and 'the contents-.of-the flask settled for about .30 minutes, after which time the lower aqueous layei was separated. The product oil was'dehazed by passage through a filter paper coalescer.

Tin-the eourseofeach run (1) immediately a black precipitate oflbS was formed-and (2) as the contacting with air continued the 'blaclc precipitate gradually disappearedandthe agent' solution'regained its normal'clear tan .color. I

Periodicallya sample of the contents of the flask was withdrawn arid thel dhazed oil tested ,for sweetness. The absence of PbS particles clearly ndicated-the completion of the.. RbS .oonue rsion reaction. It was found that the agitation ,could be..discontinued..when.about .95% of b ha beehgw ve tsdst rem inin Pbs a p e ely co ver ed -durns-..t :m n t t n P Thesour ioil -usedpnl the experiments was a low sulfur virgin heater oil distillate having the following characteristics:

Mercaptan No 16 Total sulfur (wt. percent) 0.09 ASTM distillation, F.:

Initial 315 357 50% 418 90% 505 Endpoint 549 In Table I the results of the various experiments are set out. In all the runs, the oil was doctor sweet before the PbS conversion had been completed.

Runs 1, 2 and 3 show that temperatures below 135 F. are impractically long for most commercial operation. In all the runs the two phases were very easily separated as no emulsion was present and only an extremely small amount of aqueous phase was occluded in the oil phase.

Thus having described the invention, what is claimed is:

1. A doctor sweetening process which comprises contacting a hydrocarbon oil which contains an amount of mercaptans at least sufiicient to render said oil positive to the doctor test, with doctor solution, in the presence of free-sulfur, in an amount appreciably in excess of the theoretical amount needed to convert said mercaptans to disulfides, and free-oxygen in an amount at least sufficient to oxidize the PbS formed in the sweetening reaction, at a temperature between about 140 and 200 F. for a time at least sufiicient to convert essentially all of said PbS to soluble plumbite and separating a doctor-sweet oil from said solution.

2. The process of claim 1 wherein said oil is a heater oil.

3. The process of claim 1 wherein said oil is a naphtha.

4. The process of claim 1 wherein said free-sulfur is present in an amount of between about 105% to about 200% of the theoretical requirement to convert said mercaptans to disulfides.

5. The process of claim 1 wherein said free-oxygen is present in an amount between about 150% and 250% of the theoretical requirement to convert said PbS to PhD.

6. A doctor sweetening process which comprises contacting a sour petroleum distillate with an amount of doctor solution at least suflicient to produce separate distillate and aqueous phases, in the presence of (i) an amount of free-sulfur appreciably in excess of the theoretical and (ii) an amount of free-oxygen at least sulficient to react with the PbS that is formed in the process, at a temperature between about 150 and 175 F. for a time at least suflicient to convert substantially all of said PbS to soluble plumbite and separating a sweet oil from an aqueous phase.

7. The process of claim 6 wherein said separated aqueous phase is recycled for reuse in the process.

8. The process of claim 6 wherein the time of contacting is between about 25 minutes and 10 minutes, the longer times corresponding to the lower temperatures.

9. A process which comprises (A) contacting a sour petroleum distillate boiling in the heavier-than-gasoline range with between about 1 and 20 volume percent, based on distillate, of a conventional doctor solution, in the presence of (i) between about and 200% of the theoretical requirement of free-sulfur and (ii) at least a sufiicient amount of free-oxygen to convert substantially all of the PbS formed in the process, at a temperature between about and F. for a time between about 25 minutes and 10 minutes, the longer times corresponding to the lower temperatures in order to convert substantially all of the PbS formed in the process to the soluble plumbite, and (B) separating a substantially sweet distillate from a substantially regenerated doctor solution phase.

10. The process of claim 9 wherein the solution phase of step (B) is recycled for reuse in step (A).

1-1. The process of claim 9 wherein said free-oxygen is present in an amount between about 150% and 250% of the theoretical requirement to convert said PbS to PhD.

References Cited in the file of this patent UNITED STATES PATENTS 1,789,335 Fischer et a1 Jan. 20, 1931 2,011,954 Teichmann Aug. 20,1935 2,605,165 Love July 29, 1952 2,655,482 Holtzclaw et al. Oct. 13, 1953 

1. A DOCTOR SWEETENING PROCESS WHICH COMPRISES CONTACTING A HYDROCARBON OIL WHICH CONTAINS AN AMOUNT OF MERCAPTANS AT LEAST SUFFICIENT TO RENDER SAID OIL POSITIVE TO THE DOCTOR TEST, WITH DOCTOR SOLUTION, IN THE PRESENCE OF FREE-SULFUR, IN AN AMOUNT APPRECIABLY IN EXCESS OF THE THEORETICAL AMOUNT NEEDED TO CONVERT SAID MERCAPTANS TO DISULFIDES, AND FREE-OXYGEN IN AN AMOUNT AT LEAST SUFFICIENT TO OXIDIZE THE PBS FORMED IN THE SWEETENING REACTION, AT A TEMPERATURE BETWEEN ABOUT 140* AND 200* F. FOR A TIME AT LEAST SUFFICIENT TO CONVERT ESSENTIALLY ALL OF SAID PBS TO SOLUBLE PLUMBITE AND SEPARATING A DOCTOR-SWEET OIL FROM SAID SOLUTION. 